Timber · Beams & joists
Flitch beam (timber with a steel plate) calculator
Two timber members bolted either side of a steel plate. Moments share in proportion to EI; each material is checked against its own capacity. Long-term deflection reduces the timber stiffness by the creep factor. The bolts must transfer the plate's share of the load from the timber (which receives it) to the plate: the bolt spacing follows from the bolt capacity perpendicular to the grain.
Standard: Load share by flexural stiffness · AS 1720.1 timber bending (k12 = 1, restrained by the plate) · AS 4100 plate bending (fully restrained) · creep j2 on the timber · bolts carry the plate's share
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Timber grade | F17 | |
| Each timber breadth | 45 | mm |
| Timber depth | 290 | mm |
| Plate thickness | 10 | mm |
| Plate depth | 250 | mm |
| Plate fy | 250 | MPa |
| Span | 5 | m |
| Dead load (line) | 3 | kN/m |
| Live load (line) | 4 | kN/m |
| k1 (G + Q) | 0.8 | |
| Timber φ | 0.85 | |
| Creep factor j2 | 2 | |
| Bolt design capacity ⟂ grain (double shear) | 6 | kN |
| Deflection limit L / | 300 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| EI timber / steel | 2.561 / 2.604 | ×10¹² Nmm² |
| Share of moment timber / steel | 49.6 / 50.4 | % |
| M* total | 30.0 | kNm |
| Timber M* / φMd | 14.9 / 36.0 | kNm |
| Plate M* / φMs | 15.1 / 35.2 | kNm |
| V* / φVd (combined) | 24.0 / 380.1 | kN |
| Long-term deflection | 13.4 (L/373) | mm |
| Load to transfer to the plate (ULS) | 4.84 | kN/m |
| Bolt spacing for that transfer | 1,240 | mm |
- ✓ Timber bending — 41 %
- ✓ Plate bending — 43 %
- ✓ Shear
- ✓ Deflection ≤ L/300
- ✓ Bolt spacing ≥ 4 × bolt diameter (practical ≥ 100 mm)
Method, formulas and assumptions
- Load share by EI: Mt = M·EIt/(EIt + EIs). Plate fully restrained by the timbers (Ze = 1.5Z for a compact plate, φ = 0.9).
- Timber: φ·k1·f′b·Z with k12 = 1 (lateral restraint from the bolted plate).
- Long-term deflection with the timber EI divided by j2 under G + ψlQ (ψl = 0.4) plus the short-term remainder.
- If the load is applied to the timber, the bolts deliver the plate's share w·EIs/EI; spacing = bolt capacity ⟂ grain / that load per metre.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.